The present invention relates broadly to a radar system, and in particular to a slow speed pulse chase apparatus for a bistatic radar system.
In a bistatic radar system, one of the methods of operation has the receive antenna follow the transmit pulse as it travels out into space. This can be accomplished by moving the receive antenna with its attendent beam pattern many times in very small descrete steps.
Another bistatic process utilizes many parallel receive beams that overlap all of the possible positions of the transmit pulse as it travels away from the radar system. This causes the system to be very expensive because of the large number of beams involved.
The geometry of an active illuminator and a passive receiver has been utilized in many prior art bistatic radar systems. If the receiver of a bistatic pair has a fixed beam during one transmitter interpulse period, useful echoes are only received from a small region where the transmit and receive beams overlap. The resulting coverage would be limited to a narrow fence.
In order to get a greater coverage volume for air surveillance, the receiving antenna must somehow follow the echo volume swept out by a transmitted pulse. One possibility is that the receiving antenna steer its beam along the transmit line-of-sight with a schedule that is matched to the arrival time of echoes from points along that line-of-sight. This method is called pulse chasing. The earlier paragraphs on bistatic geometry discussed the need to match the receiving beamwidth to the combination of the transmitted bandwidth and the pulse length in space.
Another method of collecting echoes from points along a transmitted out-of-sight is to produce a set of contiguous fixed beams which cover the desired segment of space along the transmitter line-of-sight. Echoes are not present continously in these beams. Instead echoes are confined to a time interval around the delay that is caused by propagation from the transmitter to the region of beam overlap and then back to the receiver. Because of this, the beams need not all be active simultaneously, and receivers can be time-shared between them.
The antenna configuration affects the basic feasibility of the alternative methods of receiving echoes from a time-varying direction, during one interpulse period. A lens-type antenna is generally compatible with a large set of fixed beams, whose selection can be programmed in time. This type of antenna has multiple output ports which correspond to different receive beam directions. An antenna which uses switched phase shifters and time delays is generally compatible with steering one beam, a monopulse group, or a small cluster of beams, to achieve pulse chasing.
FIGS. 1a, b and c show three methods of obtaining coverage along a transmitted line-of-sight by steering or switching receiving beams. The first case (prior art), shown in FIG. 1a is a pure pulse chasing system. The direction and beamwidth of a receiving beam are changed with time to follow the echo region of the transmitted pulse. This requires a rapid programmed sequence of phase shifter states in the receiving elements on the array face. FIGS. 1b and 1c are alternative embodiments of the present invention. The second case which is shown in FIG. 1b, employs a cluster of beams which span a relatively narrow sector. This cluster is fixed while the echo region sweeps through the sector, and then switched as a group to an adjacent sector. The antenna needs to have simultaneous output ports for the beams in the cluster. Switching from one sector to the next occurs at a much slower rate than does the switching for beam control in pure pulse chasing. The third case which is shown in FIG. 1c, uses a large set of adjacent beams to which receiver channels are sequentially connected. The pattern of sequencing the receivers to the beam ports causes the active receive beam closest to the transmitter to be disconnected in favor of a new active receive beam on the side of the group farthest from the transmitter. Thus, the pattern of active receiving beams is not a jumping sector (as shown in FIG. 1b), but moves like a tank track.
Pure pulse chasing (FIG. 1a) minimizes the number of receivers (one for each beam in a monopulse group, for example) and gives the simplest scheme for clocking range cells. The antenna must, however, have high-speed switching to control the moving and spreading beam. There is also some gain reduction when the beam is spread, compared to a stationary beam in a cluster. The other extreme which is represented by FIG. 1c, is compatible with a lens-type of antenna. Commutation of receivers with respect to the beam ports is needed. This introduces some complexity into the ordering of the range cells. The system of FIG. 1b has a simpler and slower beam steering, but pulse eclipsing will occur at the edges of the active sector when a jump in angle is made. That is, the cluster of beams moves before the entire pulse duration can be received, for echo points near the leading edge of the sector.
In the systems of FIGS. 1b and 1c, the shape and direction of a receive beam is fixed during the interval when it is collecting echos. The FIG. 1a system, pure pulse chasing, the beam changes during pulse reception. There are two technical issues which arise as a result. These are as follows:
1. the effect of phase shifting transients and beam rape modulation on the range sidelobes of a stretched pulse. PA1 2. the degree to which low sidelobes can be maintained when the beam is spread by introducing a non-linear phase progression across the aperture. PA1 U.S. Pat. No. 3,171,126 issued to Wiley on Feb. 23, 1965; PA1 U.S. Pat. No. 4,034,374 issued to Kruger on Jul. 5, 1977; PA1 U.S. Pat. No. 4,176,357 issued to Fales, III on Nov. 27, 1979; PA1 U.S. Pat. No. 4,214,244 issued to McKay et al on Jul. 22, 1980; PA1 U.S. Pat. No. 4,404,561 issued to Mulder et al on Sep. 13, 1983; and PA1 U.S. Pat. No. 4,408,205 issued to Hockham on Oct. 4, 1983.
In a bistatic receiving system which uses fast pulse chasing, the receiving beam moves during the reception time of an individual echo pulse. This notion can be achieved by changing the phasing of the receiving elements in the aperture. A nominal time interval between new phase conditions is one to two microseconds. In high speed pulse chasing, the beam is really stepped in short steps very frequently. The duration in any one state can be of the order of one to two microseconds. The time of the switching transient must be much shorter, of the order of 0.02 microseconds.
After a switching command to a new phase state, the phase shifter enters a transient condition whose duration depends on the intrinsic switching speed of the device. If the receiver continues to accept inputs during these transients, spurious signals and distortions can corrupt the echo pulses. An alternative is to blank the receiving channels during the interval occupied by the switching transient. This produces a known distortion of the received pulse, in the form of narrow notches occurring periodically across the pulse. In addition to the notches, the motion of the received beam axis with respect to the direction of the target causes some modulation of the pulse envelope by the gain pattern of the beam.
Patent references that are representative of the prior art and that are of interest are listed below as follows: